factorio
I've followed a few videos on train signalling for Factorio but whatever order I do the signals, both trains when in the stations will constantly say "No Path" until I remove all the signals at the junction.
Can anyone tell me how to signal this junction so both trains can use it properly? Or perhaps I need to remake the junction instead?
I've tried having normal signals coming into the junction from all sides but it doesn't seem to work.
Here's a screen shot:
Where do I put the signals?
From my tentative understanding of Factorio train mechanics, and tidbits I found on the web (I've been working on a very similar type of system recently), there are two factors here:
A train does not recalculate its path every time it is stuck, but only when it leaves a stop or it passes a regular signal. (There's also some kind of timeout or scheduled recalculation so trains don't remain stuck forever, though I think that might be newer than the version you were using.)
When a train is choosing a stop, it doesn't matter if another train is already planning to go there, but only whether the block is currently occupied.
Therefore, two trains may pick the same stop while some distance away and not notice the interference. Here's how to help them notice, at least within your Yard:
Do not allow any stacking of trains before the Yard stops. Branch the track as close to the stops as you can, and use a chain signal at the entrance to the branching. This ensures that a train will be seen to occupy the stop as soon as possible after it's actually committed to that stop.
Place a regular signal shortly before the branching point, even though that means it's right before the previously mentioned chain signal. This will ensure that the train has picked a free stop based on the latest available information.
Unfortunately, there is a case where a train is doomed to wait unnecessarily: when all stops of that name are occupied. In this case, it will always pick one and head as close to it as signals allow, then wait there until the time-based recalculation sends it elsewhere (or that one stop becomes available). The way to make the best of this is to be sure to use chain signals to prevent the train from heading down a branch before that branch's stop is available.
(Your Unload stations seem to be designed suitably already, so it might be simply that the Yard isn't efficient enough to feed them.)
Finally, one weird possibly-bogus fact that you might bump into if you expand this further: from my own testing, it seems that a train will only choose between 10 same-named stations. Any more than that, and it will only check the nearest 10. You can work around this by using tricks like having trains arriving in different directions (having a different idea of the 10 nearest), or a multi-stage choice (e.g. each yard stop goes to a different set of 10 unloading stops), which is what I'm doing.
Turns out you can see the signal values when you hover electric pylon that has signal wires on it:
Best Answer
Building train signals
Train signals are a little bit weird at first, but when you understand them it makes so much sense.
When making signals you need to know whether you want tracks to be one or two-way. I'm going to assume the following:
All tracks except for the top right ones are one-way.
When building rail signaling, here are a few things to keep in mind:
How (normal) train signals work
Every train signal defines a "block". When a train enters that block, the signal displays 'red' and other trains can't enter that block.
Because there is a train after the signal, it becomes red. Dark yellow represents the block that the signal creates.
Chain signals work the same way, except they inherit the status of the next signal. If a track splits, it checks both ways. If one of the tracks is not empty, the chain signal becomes blue, signaling the train that only one of the train tracks is empty.
How I would make this work
Lets start with the top right corner - the two-way tracks. Because trains need to be able to pass both ways, these tracks need signals on both sides. Here is what I mean (see 1):
When the train is coming from the left, the chain signal indicates which of the tracks is empty (dark and light blue block). When the train is coming from top/right, the chain signal indicates whether the (dark yellow) block is empty.
The same principle is used for other tracks, for example (see 2):
A train coming from the right can only go down. When coming back (from the left), the chain signal indicates if the (dark yellow) path is clear
Putting all of this together, we get a functioning track where trains should not crash:
If you want, you can place more rail signals on the right side of the top left track so multiple trans can wait in line to leave, but that's just aesthetics.